The invention concerns an approach to fabricating current blocking regions in a Vertical Cavity Surface Emitting Laser, VCSEL. The approach is simpler and less expensive than those used presently.
FIG. 1 is a simplified schematic of a homojunction laser 3, and is not drawn to scale. A PN junction 4 is formed between a p-plus-type body 6 of gallium arsenide, GaAs, and an n-plus-type body 9 of gallium arsenide. Metal contacts 12 provide entry- and exit paths for current 15, which is supplied by a voltage source V+. The laser produces laser output 18, which travels in a plane parallel to the junction 4. The laser will generally be situated in a resonant optical cavity, which is not shown.
FIG. 2 is a simplified schematic of a different type of laser, namely, the Vertical Cavity Surface Emitting Laser, VCSEL, labeled 30, and is also not drawn to scale. The VCSEL 30 includes a top mirror 33 and a bottom mirror 34. These mirrors are constructed of multiple layers L of dielectric material, each layer being xc2xc wavelength thick.
Current 35, indicated by the dashed arrow, flows from a metal contact 36, through a p-type region 39, through a gain region 42, through an n-type region 43, and to another metal contact 45. The gain region 42 produces light, and multiple reflections of that light between the top mirror 33 and the bottom mirror 34 induce stimulated emission of laser light, which exits the device as indicated by ray 48.
A significant feature of the VCSEL 30 is that the laser light travels perpendicular to the plane of the gain region 42, that is, perpendicular to bottom mirror 34. Gain region 42 is analogous to junction 4 in FIG. 1, in the sense that population inversion occurs in both the gain region 42 and the junction 4.
In addition, in FIG. 2, the light which stimulates emission of photons within the gain region 42 bounces between the top mirror 33 and the bottom mirror 34. However, stimulated emission only occurs within the gain region 42. The thickness T of the gain region 42 is very small, of the order of a few hundred angstroms, and is much smaller than the corresponding distance Ti in Figure Thus, since stimulated emission in FIG. 2 only occurs along a relatively small thickness T, losses must be reduced to a minimum. One source of loss is scattering which would occur at the edge 50 of the top mirror 33. To reduce this loss, current-blocking regions 53 are fabricated. They block current from flowing near the edge 50. The absence of current means that photon generation is absent, so that stimulated emission is also absent, at that location.
Fabrication of the current-blocking region 53 is expensive, or at least complex. In one approach, ion implantation is used, wherein the p-type region 39 in FIG. 3 is bombarded by high-velocity ions, indicated by dashed arrows 54. These ions bury themselves beneath the surface 55 and generate the current-blocking region 53 in FIG. 4. Region 53 is generated because the ions 54 compensate the p-type dopants (not shown), effectively converting region 53 into an intrinsic semiconductor, which is low in conductivity, at least at room temperature.
However, this ion implantation technique requires strict process control in order to develop the proper profile 65 in plot 68 in FIG. 4. Plot 68 indicates ion concentration, as a function of depth in the p-type layer 39. Also, the overall process requires later annealing of the structure, after the implantation.
In another approach, current blocking region 53 is fabricated through lateral oxidation, wherein the oxidation is begun at regions 70 in FIG. 3, and invades the p-layer 39 as indicated by arrows 73. However, the lateral oxidation process is difficult to control.
In a third approach, shown in FIG. 5, a p-type layer 80 in structure A, at the upper left of the Figure, is etched away to form the mesa 83 in structure B. Then, in structure C, the current blocking layer 53 is fabricated, by implantation or surface oxidation. (Intermediate steps required for generation of layer 53 are not indicated.) Next, the p-type layer is expanded in size through crystal regrowth into body 39, as in Structure D. After that, known process steps are implemented to produce the final structure Z.
However, the processing steps required to convert structure C into structure D are expensive and complex. Specifically, the p-type layer 39 in structure D, as well as the gain region 42, must all consist of a monocrystalline body of material. Adding a monocrystalline body to the p-layer 83 shown in structure C, to create structure D, is a complex process, as is crystal regrowth generally, which is the process used.
The Inventors have developed a process for producing the current blocking region 53 in FIG. 2, but in a simpler manner than described above.
Numerous textbooks exist on laser technology. A good simplified treatment is found in Optoelectronics. An Introduction, by Wilson and Hawkes, Third Edition (Prentice Hall, 1998, ISBN 0-13-103961-X). This book is hereby incorporated by reference, partly to show, in simplified terms, the present state of the art.
In one form of the invention, a film of gold is positioned across the optical gain path of a VCSEL. The gold film delivers electrical current into the semiconductor material within the gain path, and eliminates the need for a crystal re-growth step.